My advice would be take to the train
but to book on line as smaller groups to obtain the best fares. I understand that you would prefer to sit together but in any group of 20 there will be group dynamics and the large group will split into smaller groups anyway. It will be up to you as group leader* to allocate the seat reservations and smooth out any difficulties en route - I wouldn't really foresee any.
With regard to flying over taking the train for Edinburgh - London (& intermediate stops) I personally feel that the train wins hands down nearly every time (I only fly when I have a connecting flight from a London Airport these days) - when comparing you do have to take in the complete door to door timings into consideration not to mention the extended check in times at the airport these days.
I must, however, also state that I am not always impressed with the groupsave deals that I see from time to time. One example will suffice:-
e.g. A coach party [approx 30 pax] boarded at an unstaffed station on a Saturday before 09:15. One third of the party held senior rail cards. As the route became circular they wished to travel the full circle without alighting back to their originating station.
The tour organiser contacted the railway's group travel department to find the best fare. I will summarise the advice given 1) A 10% group discount would be applied BUT on the SDR fare as it was before 09:15, 2) The senior railcard holders were advised to forgo their discount so everybody could get a 10% reduction, & 3) The tickets were issued to the furthest station on the circle.
In my view there were three compound errors 1) As it was Saturday off peak tickets were valid all day on that flow - the
CDR should have been the base fare, 2) To maximise the discount the senior railcard holders should have been taken out the equation - 20 pax are enough to obtain the standard 10% group discount. If, and this would be a internal matter for the coach party, they wanted to equate the fares then by simply adding the combined [group + 10x senior RC fare] amount and dividing by 30 this would be achieved, & 3) a moot point but as the party were not alighting en route then the fare should have been offered to the first stop past the junction where the circular route diverges
not the furthest.
The passengers would have been cheaper buying their tickets from me on the train! However the adverts and warnings mislead passengers into sometimes making expensive mistakes. In the above case, although there were three compound errors, I do not think the
office was trying to overcharge the passengers - it simply comes down to training & experience. If you do decide to take the train again (& I urge you to give it a second chance) then do come back to the site for advice if you think the fares quoted are too dear. Experts on here - not me sadly - will try to get your fare down and if not explain why.
*As group leader it is surely allowed to book yourself into 1st Class as this will give you a relaxed journey far from the madding crowd. This can be achieved by combining your 1st fare with the various small group fares and dividing by the total number of passengers. Therefore everyone pays a small premium for all your hard work in sorting out their travel arrangements. Of course you will have to appoint a deputy to oversee your colleagues in steerage but that's what leadership is all about. 